Spatial ability is one of the few intellectual skills in which evidence consistently indicates a sex difference in performance: When a task involves imagining what an object will look like when it is rotated in space, males' performance typically surpasses that of females (Maccoby & Jacklin, 1974). Despite the consistency with which this difference is found, there is little evidence regarding the psychological processes that underlie the difference. That is, solving problems like those appearing on tests of spatial ability involves a series of processes, and sex differences in performance on spatial tasks might be attributable to any of these processes. The goal of the proposed research is to identify these processes. The task used is similar to problems appearing on tests of spatial ability: Subjects decide, as rapidly as possible, if two stimuli presented in different orientations are the same or different. By computing the function relating response time to the difference in orientation between the two stimuli, total response time can be broken down into the times necessary to encode, rotate, and compare stimuli. The general aim of the proposed research is to evaluate encoding, rotation, and comparison processes as possible loci of sex differences in spatial ability. Other specific goals include (a) determining the effect of the familiarity of stimuli on the magnitude of sex differences, (b) determining if the sex difference is unique to tests of visual-spatial processing, or if it also occurs in other modes, such as haptic processing, (c) determining if males and females use the same algorithms for mentally rotating stimuli and (d) identifying the developmental course of the above phenomena.